The 17-year-old player was still among the top 10 on the leaderboard in the final round until faltering down the stretch Sunday. He closed with two double bogeys and two bogeys.
"I was humbled by this back nine," Spieth said. "I'm focused on college now. Had I won today, who knows, but I'm focused now on the next step. This will sink in, and I'm sure it will sink in in a positive way. But it stings right now, it really does."
Spieth, who played his final round with eventual champion Keegan Bradley, made the cut at the Nelson for the second straight and had a closing 7-over 77.
At 6 over for the tournament, Spieth tied for 32nd. He was nine strokes behind Bradley (68), who beat Ryan Palmer in a playoff. Spieth's total was 10 strokes higher than last year's score at TPC Four Seasons when he finished tied for 16th.
Spieth (pronounced SPEE-th) missed his high school graduation ceremony Saturday. His third round ended about the same time the ceremony for the other Dallas Jesuit Prep seniors was getting started about 20 miles from the course.
On another windy day Sunday at TPC Four Seasons, Spieth had four birdies and eight over-par holes (five birdies and three double bogeys).
Spieth had a lot of birdies this weeks, but also a lot of bogeys that kept him from possibly contending to be the first amateur to win a PGA Tour event since Phil Mickelson at the 1991 Northern Telecom Open. The youngest amateur winner is still Johnny McDermott, who was 19 years, 10 months when he won the 1911 U.S. Open.
The 20 birdies by Spieth this week were the second-most subpar holes behind the 22 by defending champion, who finished fifth. But Spieth also had 20 holes over par with 14 bogeys and six double bogeys.
"I knew I would make a few birdies but, you know, it's hard right now to take it. It's hard to take it right now finishing 6 over on my last four holes," he said. "It wasn't the nerves, I just didn't hit good shots. Just plain didn't hit good shots."
When Spieth made it to play the weekend rounds at the Nelson last year as a 16-year-old, he was then the sixth-youngest person to make the cut in a PGA Tour event.
The 2009 U.S. Junior Amateur champion, who plans to attend the University of Texas this fall, is scheduled to play in a U.S. Open qualifying tournament June 6 in Dallas.
"I had an awesome time this week, just like I always do," he said. "I don't think I surprised myself having a chance again. I know this course really well and prepared and I was ready to go."